C-NOT gate based on ultracold Rydberg atom interactions

The Rydberg states of neutral atoms are strongly polarisable and possess long lifetimes because of high energies which can lead to strong and long range dipole-dipole interactions.The energy levels corresponding to these states are shifted because of dipole-dipole interactions and can be used to block transitions of more than one excitation in the Rydberg regime.This reputed Rydberg blockade is obtained when the excitation is shifted out of resonance by these interactions.Electromagnetically induced transparency(EIT)is sensitive to a small detuning.At large distances,up to several micrometers,the interactions can interrupt the EIT consequence.Herein we investigate a novel scheme based on EIT and Rydberg blockade and performed a simulation of a controlled-NOT(C-NOT)quantum gate which is critical for quantum computation by using neutral atoms.     

Fast quantum gates for neutral atoms

We propose several schemes for implementing a fast two-qubit quantum gate for neutral atoms with the gate operation time much faster than the time scales associated with the external motion of the atoms in the trapping potential. In our example, the large interaction energy required to perform fast gate operations is provided by the dipole-dipole interaction of atoms excited to low-lying Rydberg states in constant electric fields. A detailed analysis of imperfections of the gate operation is given.     

中性原子量子计算研究进展

相互作用可控、相干时间较长的中性单原子体系具备在1 mm2的面积上提供成千上万个量子比特的规模化集成的优势,是进行量子模拟、实现量子计算的有力候选者.近几年中性单原子体系在实验上取得了快速的发展,完成了包括50个单原子的确定性装载、二维和三维阵列中单个原子的寻址和操控、量子比特相干时间的延长、基于里德伯态的两比特量子门的实现和原子态的高效读出等,这些工作极大地推动了该体系在量子模拟和量子计算方面的应用.本文综述了该体系在量子计算方面的研究进展,并介绍了我们在其中所做的两个贡献:一是实现了"魔幻强度光阱",克服了光阱中原子退相干的首要因素,将原子相干时间提高了百倍,使得相干时间与比特操作时间的比值高达105;二是利用异核原子共振频率的差异建立了低串扰的异核单原子体系,并利用里德伯阻塞效应首次实现了异核两原子的量子受控非门和量子纠缠,将量子计算的实验研究拓展至异核领域.最后,分析了中性单原子体系在量子模拟和量子计算方面进一步发展面临的挑战与瓶颈.     

Fast Rydberg gates without dipole blockade via quantum control

We propose a scheme for controlling interactions between Rydberg-excited neutral atoms in order to perform a fast high-fidelity quantum gate. Unlike dipole-blockade mechanisms already found in the literature, we drive resonantly the atoms with a state-dependent excitation to Rydberg levels, and we exploit the resulting dipole-dipole interaction to induce a controlled atomic motion in the trap, in a similar way as discussed in recent ion-trap quantum computing proposals. This leads atoms to gain the required gate phase, which turns out to be a combination of a dynamic and a geometrical contribution. The fidelity of this scheme is studied including small anharmonicity and temperature effects, with promising results for reasonably achievable experimental parameters.     

Implementation of quantum phase gate between two atoms via Rydberg antiblockade and adiabatic passage

Combining adiabatic passage and Rydberg antiblockade, we propose a scheme to implement a two-qubit phase gate between two Rydberg atoms. Detuning parameters between frequencies of atomic transitions and those of the corresponding driving lasers are carefully chosen to offset the blockade effect of two Rydberg atoms, so that an effective Hamiltonian,representing a single-photon detuning L-type three-level system and concluding the quantum state of two Rydberg atoms excited simultaneously, is obtained. The adiabatic-passage technique, based on the effective Hamiltonian, is adopted to implement a two-atom phase gate by using two time-dependent Rabi frequencies. Numerical simulations indicate that a high-fidelity two-qubit p-phase gate is constructed and its operation time does not have to be controlled accurately. Besides,owing to the long coherence time of the Rydberg state, the phase gate is robust against atomic spontaneous emission.     

稀薄里德伯原子气体中的两体纠缠

量子纠缠是量子信息处理和量子计算中不可或缺的物理资源,制备稳定可操控的量子纠缠是研究的热点之一.里德伯原子具有不同于普通中性原子的特点,长寿命和原子之间强烈的偶极相互作用,使得它成为量子信息处理和量子计算的最优候选者.本文在稀薄里德伯原子气体中,构建了空间四面体排布的里德伯原子模型(空间等距的四个原子模型),通过数值求解主方程来研究两体纠缠和里德伯激发的稳态和瞬态动力学性质,发现偶极阻塞机制下的量子纠缠最大,其他满足反偶极阻塞条件的高阶激发引起的纠缠较小,进而从理论上分析了这两种机制下量子纠缠的物理实质.     

Optimized nonadiabatic holonomic quantum computation based on Förster resonance in Rydberg atoms

In this paper, we propose a scheme for implementing the nonadiabatic holonomic quantum computation (NHQC+) of two Rydberg atoms by using invariant-based reverse engineering (IBRE). The scheme is based on Förster resonance induced by strong dipole–dipole interaction between two Rydberg atoms, which provides a selective coupling mechanism to simply the dynamics of system. Moreover, for improving the fidelity of the scheme, the optimal control method is introduced to enhance the gate robustness against systematic errors. Numerical simulations show the scheme is robust against the random noise in control fields, the deviation of dipole–dipole interaction, the Förster defect, and the spontaneous emission of atoms. Therefore, the scheme may provide some useful perspectives for the realization of quantum computation with Rydberg atoms.     

A concise review of Rydberg atom based quantum computation and quantum simulation

Quantum information processing based on Rydberg atoms emerged as a promising direction two decades ago.Recent experimental and theoretical progresses have shined exciting light on this avenue.In this concise review,we will briefly introduce the basics of Rydberg atoms and their recent applications in associated areas of neutral atom quantum computation and simulation.We shall also include related discussions on quantum optics with Rydberg atomic ensembles,which are increasingly used to explore quantum computation and quantum simulation with photons.     

Experimental demonstration of switching entangled photons based on the Rydberg blockade effect

The long-range interaction between Rydberg-excited atoms endows a medium with large optical nonlinearity. Here, we demonstrate an optical switch to operate on a single photon from an entangled photon pair under a Rydberg electromagnetically induced transparency configuration. With the presence of the Rydberg blockade effect, we switch on a gate field to make the atomic medium nontransparent thereby absorbing the single photon emitted from another atomic ensemble via the spontaneous fourwave mixing process. In contrast to the case without a gate field, more than 50% of the photons sent to the switch are blocked,and finally achieve an effective single-photon switch. There are on average 1-2 gate photons per effective blockade sphere in one gate pulse. This switching effect on a single entangled photon depends on the principal quantum number and the photon number of the gate field. Our experimental progress is significant in the quantum information process especially in controlling the interaction between Rydberg atoms and entangled photon pairs.     

A Scheme for Atomic Entangled States and Quantum Gate Operations in Cavity QED

We propose a scheme for controllably entangling the ground states of five-state W-type atoms confined in a cavity and realizing swap gate and phase gate operations. In this scheme the cavity is only virtually excited and the atomic excited states are almost not occupied, so the produced entangled states and quantum logic operations are very robust against the cavity decay and atomic spontaneous emission.     

Implementation of Controlled Geometric Phase Gate for Two Trapped Neutral Atoms

We propose a scheme for realizing a controlled geometric phase gate for twoneutral atoms. We apply the stimulated Raman adiabatic passage to transferatoms from their ground states into Rydberg excited states, and use theRydberg interaction induced energy shifts to generate geometric phase andconstruct quantum gates.     

Optical lattices,ultracold atoms and quantum information processing

We review novel methods for the investigation, control and manipulation of neutral atoms in optical lattices. These setups allow unprecedented quantum control over large numbers of atoms and thus are very promising for applications in quantum information processing. After introducing optical lattices we discuss the superfluid (SF) and Mott insulating (MI) states of neutral atoms trapped in such lattices and investigate the SF-MI transition as observed experimentally recently. In the second part of the paper we give an overview of proposals for quantum information processing and show different ways to entangle the trapped atoms, in particular the usage of cold collisions and Rydberg atoms. Finally, we discuss briefly the implementation of quantum simulators, entanglement enhanced atom interferometers, and ideas for robust quantum memory in optical lattices.     

Transfer of entangled state, entanglement swapping and quantum information processing via the Rydberg blockade

We provide a scheme with which the transfer of the entangled state and the entanglement swapping can be realized in a system of neutral atoms via the Rydberg blockade. Our idea can be extended to teleport an unknown atomic state. According to the latest theoretical research of the Rydberg excitation and experimental reports of the Rydberg blockade effect in quantum information processing, we discuss the experimental feasibility of our scheme.     

长程铯里德堡分子的势能曲线

本文介绍了半经典近似下的低能电子-原子散射理论, 引入贋势描述里德堡电子与基态原子的相互作用, 数值计算了铯原子nS (n=30-60)里德堡态与6S基态原子形成的长程里德堡分子的势能曲线. 并对最外层势阱进行分析, 获得长程里德堡分子的势阱深度、平衡距离与主量子数n的关系. 为实验制备里德堡分子并进一步分析其性质提供理论依据. 里德堡分子对外界非常敏感, 可用于微弱信号的检测.     

Rydberg quantum computation with nuclear spins in two-electron neutral atoms

Alkaline-earth-like (AEL) atoms with two valence electrons and a nonzero nuclear spin can be excited to Rydberg state for quantum computing. Typical AEL ground states possess no hyperfine splitting, but unfortunately a GHz-scale splitting seems necessary for Rydberg excitation. Though strong magnetic fields can induce a GHz-scale splitting, weak fields are desirable to avoid noise in experiments. Here, we provide two solutions to this outstanding challenge with realistic data of well-studied AEL isotopes. In the first theory, the two nuclear spin qubit states |0〉 and |1〉 are excited to Rydberg states |r〉 with detuning Δ and 0, respectively, where a MHz-scale detuning Δ arises from a weak magnetic field on the order of 1 G. With a proper ratio between Δ and Ω, the qubit state |1〉 can be fully excited to the Rydberg state while |0〉 remains there. In the second theory, we show that by choosing appropriate intermediate states a two-photon Rydberg excitation can proceed with only one nuclear spin qubit state. The second theory is applicable whatever the magnitude of the magnetic field is. These theories bring a versatile means for quantum computation by combining the broad applicability of Rydberg blockade and the incomparable advantages of nuclear-spin quantum memory in two-electron neutral atoms.     

Quantum computation based on a quantum switch in cavity QED

A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating “bus”; thus, the effect of atomic spontaneous emission can be neglected and the strict requirements for the cavity can be relaxed.     

Two quantum oscillators coupled with a planar radio frequency ion trap

In this scheme,two quantum oscillators in a planar radio frequency ion trap are coupled by the trap electrodes.The ions motional states encode the quantum bits (qubits),and a swap gate could be achieved.Under different conditions of the experiments,the intensity of the coupling between two quantum oscillators and the dissipation of the system are calculated.We compute fidelities for a quantum swap gate and discuss experimental issues.     

基于非均匀外场的双量子比特自旋XYZ模型的swap门操作

自旋模型在实现量子信息处理中起着很重要的作用.将自旋之间的海森堡相互作用作为最基本量子比特之间的相互作用,将会使量子比特之间产生量子纠缠;同时忽略掉自旋之间相互作用的各向异性可直接用来实现(swap)n量子门,再配以单量子比特旋转门,可构成完备的量子计算基本门.事实上,各向异性相互作用是存在于任何固态材料中的,其对实现量子逻辑门的影响很值得研究.本文讨论了在非均匀外场下XYZ模型的双量子比特swap门的实现问题,给出非均匀外场以及各向异性相互作用导致的swap门操作的误差.     

Capacitive coupling of atomic systems to mesoscopic conductors

We describe a technique that enables a strong, coherent coupling between isolated neutral atoms and mesoscopic conductors. The coupling is achieved by exciting atoms trapped above the surface of a superconducting transmission line into Rydberg states with large electric dipole moments that induce voltage fluctuations in the transmission line. Using a mechanism analogous to cavity quantum electrodynamics, an atomic state can be transferred to a long-lived mode of the fluctuating voltage, atoms separated by millimeters can be entangled, or the quantum state of a solid-state device can be mapped onto atomic or photonic states.     

Shortcut-to-Adiabatic Controlled-Phase Gate in Rydberg Atoms

A shortcut-to-adiabatic protocol for the realization of a fast and high-fidelity controlled-phase gate in Rydberg atoms is developed. The adiabatic state transfer, driven in the high-blockade limit, is sped up by compensating nonadiabatic transitions via oscillating fields that mimic a counterdiabatic Hamiltonian. High fidelities are obtained in wide parameter regions. The implementation of the bare effective counterdiabatic field, without original adiabatic pulses, enables to bypass gate errors produced by the accumulation of blockade-dependent dynamical phases, making the protocol efficient also at low blockade values. As an application toward quantum algorithms, how the fidelity of the gate impacts the efficiency of a minimal quantum-error correction circuit is analyzed.